Possible Structural Distortions in NiGa$_{2}$ S$_{4}$ indicated by T-dependent Raman Modes*

NiGa$_{2}$S$_{4}$ contains two dimensional sheets of spin-1 Ni$^{2+}$ ions arranged in a triangular lattice where ferromagnetic nearest neighbor interactions and antiferromagnetic third nearest neighbor interactions lead to magnetic frustration which suppresses three dimensional magnetic ordering above 1.5K [1]. We studied structural distortions in NiGa$_{2}$S$_{4}$ by Raman spectroscopy on single crystals in the energy range of 150 cm$^{-1}$ to 500 cm$^{1}$. For temperatures below 300K the 446 cm$^{-1}$ A$_{\mathrm{1g}}$ mode splits and additional E$_{\mathrm{g}}$ modes are observed between 250 cm$^{-1}$ and 450 cm$^{-1}$. These high energy features are associated with sulfur vibrations but are not predicted to occur on the basis of the point group symmetry inferred from x-ray diffraction. We discuss possible lattice distortions due to magneto-elastic coupling and their potential effects on low temperature frustrated magnetism. \\[4pt] [1] C. Stock et al., Phys. Rev. Lett. 105, 037402 (2010)